The present invention relates to an improved window for a gallium nitride (GaN)-based light-emitting diode (LED).
A semiconductor light-emitting diode (LED) includes a substrate, a light emitting region, a window structure, and a pair of electrodes for powering the diode. The substrate may be opaque or transparent. Light-emitting diodes which are based on gallium nitride (GaN) compounds generally include a transparent, insulating substrate, i.e., a sapphire substrate. With a transparent substrate, light may be utilized from either the substrate or from the opposite end of the LED which is termed the xe2x80x9cwindowxe2x80x9d.
The amount of light generated by an LED is dependent on the distribution of the energizing current across the face of the light emitting region. It is well known in semiconductor technology that the current flowing between the electrodes tends to concentrate in a favored path directly under the electrode. This current flow tends to activate corresponding favored portions of the light-emitting region to the exclusion of portions which fall outside the favored path. Further since such favored paths fall under the opaque electrode, the generated light reaching the electrode is lost. Prior art GaN LEDs have employed conductive current spreading layers formed of nickel/gold (Ni/Au), and have a gold (Au) window bond pad mounted on such layers. In such arrangements, the Ni/Au layer and/or the Au bond pad tend to peel during the wire bonding operation to the pad.
In one embodiment consistent with the present invention, light is utilized at the output of the window structure, which includes a very thin, semi-transparent nickel oxide/gold (NiOx/Au) contact layer formed on a p-doped nitride compound window layer; a semi-transparent amorphous conducting top window layer; and a p electrode structure formed of a titanium layer with a covering Au bond pad. The amorphous top layer, by way of example, may be formed of indium tin oxide (ITO), tin oxide (TO), or zinc oxide (ZnO). Layers of other amorphous, conductive, and semi-transparent oxide compounds also may be suitable for construction of the top window layer.
Advantageously, the thin NiOx/Au layer provides an excellent ohmic connection to both the amorphous current spreading conducting layer and to the magnesium (Mg)-doped GaN window layer. The highly conductive amorphous layer efficiently spreads current flowing between the electrodes across the light-emitting region to improve the efficiency of the device.
Additionally, the titanium electrode passes through both the amorphous conducting layer and the underlying Ni/Au to: (a) form an ohmic contact with those layers; (b) contact the p-doped top widow layer and form a Schottky diode connection therewith; and (c) provide good adhesion between the titanium (Ti) and the magnesiusm (Mg)-doped window layer. The Schottky diode connection forces current from the electrode into the amorphous conducting layer and eliminates the tendency of the prior art structures to concentrate current in a path directly under the electrode.